Speed Control Motors; Servo, AC Induction, or Brushless (BLDC) Too Valuable To Keep Secret: The Power of Brushless Motors When it comes to speed control, you may think the choice is between an inverter driven three-phase motor or servo motor, but did you know that brushless motors specialize in speed control? The brushless motor is growing in popularity in the world of speed control. However, not everyone is familiar with the power and performance to cost aspects of the brushless motor. Even in usage situations where it has demonstrated its capabilities, it tends to lag behind the two leading players, the inverter driven three-phase motor and the servo motor. This article introduces the capability of brushless motors, as well as points to consider when choosing a compact speed control motor. Inverter or servo motor: Which should be used to control speed?
BMU Series Speed Control Brushless Motor with Spur Gear Head
The servo motor stabilizes speed and solves the problem of synchronization of multiple conveyors, but taking cost into consideration, compromises in your setup would almost certainly be necessary. Is there any product which is easy to use and which does not lead to increased costs?
When it comes to speed control, the common choice is a three-phase induction motor that controls speed by use of a general-purpose inverter. For many, this may be the natural choice because it allows you to freely set a temporary driving speed that you can change in the future. Indeed, the inverter is commonly used, but does it actually meet basic needs or solve common complaints?
A hassle-free way to synchronize the speed of two motors
• Is it convenient to change the speed every time the task (weight or viscosity) changes? • What happens if you want to run the conveyor at high speed while transporting products, and then at a low speed during inspection?
When the speed of the conveyor is inconsistent
• When synchronizing the speed of more than two conveyor lines, the speed must be adjusted – can this operation be done simply? If speed, torque, or improved controls are needed, upgrading to a servo motor is certainly an option. However, considering the relatively low cost of the inverter driven three-phase motor, by changing to a servo motor you will face the problem of increasing expenditure.
To mix at a constant speed
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AC Induction Motor- Speed/ Torque Chart To run the conveyor at a high speed while transporting products, but slowly during inspection
Actual speed when the maximum permissible torque is applied
Torque [N•m]
0.4
Permissible torque
0.3 0.2
Speed when no load is applied
0.1 0
0
500
1300 1500
2000
2500
Speed [r/min]
In fact, there is an effective third alternative. The brushless motor is similar in structure to a servo motor, however specifically designed for speed control.
Figure 1 When Load is Applied, Speed Decreases
Also, because torque is lower in an AC motor at high or low speeds from rated speed, which is an inherent torque characteristic of the three-phase motor, it is difficult to obtain both the speed and torque you want at the same time. The inverter is effective when the operation continues at a fixed speed, but it is not ideal for multispeed operations. Heat is also a common component to AC induction motors. To combat this a cooling fan is attached to the back of the motor. Due to the AC induction motor’s design, heat rises when the motor runs slow and the cooling fan is runs slower. Inversely, when the motor is running at high speed, heat from the windings is also increasing.
Servo motor performance at inverter prices: Brushless motors, the third alternative The position of the brushless motor is, in simple terms, between the inverter and the servo motor. It is a motor dedicated to speed control that controls speed as effectively as a servo motor for a lower price, closer to that of an inverter. Let us explain more. ◊ Open-loop system: AC induction motor disadvantages of control by inverter
Most AC induction motor inverters do not communicate with the motor, however recently with additional encoders or analog signal devices added separately to the motor or moving parts, this is becoming an option. The drawback is the additional costs and tuning to a typically low cost solution. The majority of today’s inverters still run open loop. Under the open-loop system, when the load changes, the actual speed does not follow the command. This is why the speed changes (slower when more load is added) and depending on the load, why speed synchronization over multiple axes is difficult.
◊ Closed-loop control: Servo motors and Brushless motors, excellent speed characteristics
Both the servo motor and the brushless motor adopt a PM motor (permanent magnet is used for the rotor) and come standard with closed-loop speed control, where the motor’s operation status is fed back to the driver. This ensures the motor speed remains constant at the commanded level and enables the speed of two motor axes to be synchronized. Additionally, flat torque is produced whether operating at high or low speed. Even if the load changes, at whatever speed is commanded, stable driving speed is ensured. This means these two motors are highly effective in situations in which the inverter struggles. 2
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Servo & Brushless Motors- Speed/ Torque Chart
As for the brushless motor, its function focuses on speed control. If position control is not necessary, speed can be controlled with sufficient accuracy by a Hall Effect IC (sensor). Hall Effect ICs are simpler and less expensive than encoders. In short, since they specialize in speed control, brushless motors cost much less than the all-round servo motor. This is the same for a speed reducer (gear head) – a vital component of conveyor lines. In the brushless motor, the standardized spur gear, which is small and in many situations the same as that used with the three-phase motor, can be used.
Starting torque
Torque [N•m]
0.573 0.450 0.382
Rated torque
Short-term operation area Limited Duty Region
0.287 Continuous Continuousoperation Duty Region area
0
80
1000
2000
3000
4000
Speed [r/min] • Torque is not lost even at low speeds • Even if the load changes, operation speed is kept constant
Figure 2
Wide Speed Ratio from High to Low Speeds with Flat Torque
An expensive all-rounder versus an inexpensive dedicated motor
Motor
Spur Gearhead
◊ The difference between servo motors and brushless motors:
+
Needless to say, the servo motor is different from the brushless motor. Generally speaking, the difference is that one is capable of high performance and all around speed control, while the other is dedicated solely to speed control.
Figure 4
In the case of the servo motor, which is able to perform high-accuracy positioning and high-precision speed, reducers (gear head) with the planetary gear mechanism are commonly employed. This naturally increases the cost of the motor.
Encoder monitors operation • Speed control • Position control • Torque control
Servo motor
Both motors offer closed-loop control with motor operation feedback Specialization in speed control
Brushless Motors Use Cost Effective Spur Gearheads
Brushless motor
Servo motor (200 W, with gear)
Hall Effect IC (sensor) monitors operation
• Controls speed, position, and torque • Highly functional, high performance • Comparatively expensive
• Stable speed control close
Figure 3 The Brushless Motor Structure and Characteristics are Similar to Those of the Servo Motor
Brushless motor to that of the servo motor (200 W, with gear) • Comparatively inexpensive
In the servo motor, an encoder is mounted to the motor, making highly precise and accurate position, speed, and torque control possible. It is a highly efficient all-round motor. An accurate encoder is a very expensive part of the motor, although this depends on the encoder’s resolution capability.
Inverter • Speed change by (for 200 W) adjusting frequency + • Comparatively inexpensive 3-phase motor (200 W, with gear)
Standard Price:
$1,500.00
Standard Price:
$441.00
Standard Price:
$374.00
• Prices are taken from websites (verified by Oriental Motor)
Figure 5
3
Brushless Motors are Similarly Priced to the Inverter
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Features of the brushless motor other than speed control: No tuning required, compact motor size, energy saving
Brushless motor drivers also tend to be very simple to interface with, staying true to the motor concept of no tuning. Controlling speed can be as simple as turning a speed potentiometer or as friendly as analog 0-10 VDC from a PLC or host system. Many brushless motor drivers are now coming with built in communication, allowing for common platform network communications to be an option. While we briefly touched on this earlier, a significant advantage to the brushless motor offering is the use of lower cost spur reducer (gear heads). Because there is no high precision positioning required or the need for expensive bearings to handle moment loads or high thrust loads, brushless motors commonly use the same type of gears found on the AC induction motor. This allows for easy upgrading and requires no maintenance. As reducing space, especially on conveyor lines is important, using parallel spur reducers is common. Where more torque may be required, new hollow shaft flat spur reducers are available, increasing the permissible torque significantly. We have put together a summary (Table 1) that compares these three technologies in a simple to understand format. We have also included a review of the brushless motor advantages called “Five minute guide to the basics of brushless motors”. In summary, while Brushless motors are not new to the market, their advantages are becoming better well known as more and more OEM are selecting brushless motors over AC induction motors with inverters. While servo motors offer many performance functions, if speed control is all that is required, brushless motors are worth evaluating.
Highly accurate speed control at a reasonable price is not the only advantage of the speed control Brushless motor. 50.4 (1.98 in.)
−84 mm (3.31 in.)
Output Power times
Figure 6
90 (3.54 in.)
1.3
Inverter Control Three-Phase Induction Motor 90 W (1/8 HP) Mass: 3.2 kg (7.0 lb.)
90 (3.54 in.)
BMU Series Brushless Motor 120 W (1/6 HP) Mass: 1.2 kg (2.6 lb.)
135 (5.31 in.)
Brushless Motor is More Compact Compared to the AC Induction Motor
Compared with the invertercontrolled three-phase motor, the brushless motor is thinner and features higher torque. Since a permanent magnet is used in the rotor, the motor is thinner and features higher power. Compared with a three-phase induction motor with a frame size of 3.54 in. (90 mm), a brushless motor of the same specifications is 3.31 in. (84 mm) shorter and offers 1.3 times more output power. This compact motor structure allows you to downsize your equipment. Since the brushless motor is dedicated to speed control, it can be used right away without going through the parameter setup or adjustment which is required for inverters and servo motors, saving time and effort. Brushless motor drivers are commonly available in AC or DC voltage input. This allows for greater design flexibility. For example, if a machine is used in many different countries where power input is different from country to country, a DC input is the desired choice, or if 24-48 VDC battery input is required. If simple AC input is required, whether 115 VAC or 220-230 VAC single or three phase, these common input voltage drivers are available. 4
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Comparison of small-size industrial speed control motors Inverter + Three Phase Motor Three-phase induction motor + General-purpose inverter Composition / Structure / System
Brushless Motor
Servo Motor
Sensor mounted to magnet motor (SPM type) + Dedicated driver
Encoder is mounted to magnet motor (SPM type/ IPM type)
The same as the servo motor except that Hall Effect IC (sensor)
Encoder detects rotor position, more accurately than sensor. Hall Effect IC (sensor) / encoder
Inverter
Motor
Motor
Driver
Open-loop control
Closed-loop control
Control Function
Speed control with accuracy not required
Speed control (Torque control is partly possible)
Speed control, position control, torque control
Rotation Speed (speed ratio)
100–2400 r/min (1:24)
80–4000 r/min (1:50)
Up to 5500 r/min
When load is applied, speed slows down
Starting torque
Actual speed when the maximum permissible torque is applied
Permissible Permissable torque
0.3 0.2
Speed when no load is applied
0.1 0
0
500
1300 1500
Speed [r/min]
2000
Rated torque
Limited Duty Region Short-term operation area
Torque [N•m]
Torque
Torque [N•m]
0.4
Continuous Continuousoperation Duty Region area 0
100
1000
2000
3000
200 W ~ $374 Comparatively inexpensive
200 W ~ $441 Less expensive than servo motor
Motor Exterior
Induction motor
The same mounting as induction motor. Length (depth on the size of motor) is very short.
Efficiency / Energy Saving Performance
Efficiency of induction motors is not high.
Suitable Operations
4000
Speed [r/min]
2500
Price for Reference: Motor (w/ gear) + Driver
Speed Regulation (load)* Responsiveness Overrun
Even if load is applied, consistent torque and speed are maintained.
200 W ~ $1,500 Comparatively expensive to other control motors
(Different depending on the accuracy and output of encoder)
Mounting is small for its output. Length (depth on the size of motor) is very long.
High efficiency thanks to permanent magnet motor
-3 ~ -15%
±0.2 ~ ±0.5%
±0.01%
Low Yes, Large variations
High Yes, Large variations
High Performs highly accurate positioning
• The main use is for operation at • When speed changes, torque and speed a fixed speed are kept stable. • When speed needs to be changed • Multi-speed operation quickly
• Highly-responsive and high-precision positioning, speed control, and torque control • Multi-speed operation
Table 1
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Five-minute guide to the basics of brushless motors ◊ Structure and Principle
Rotor Hall Effect IC (Sensor)
The brushless motor controls speed in a closed-loop system. Based on the signal detected by the Hall Effect IC (sensor) mounted to the motor, the transistor in the drive circuit turns on and off, and the motor rotates. It is generally called a “brushless DC motor.” This name reflects the background of the way it was created – the mechanical contact between the brush and commutator, which was a weak point of the DC motor, was replaced by electrical treatment, and maintenance became unnecessary.
Stator
Motor
Brushless motor
Driver Power supply
Speed potentiometer
Converter section
Inverter section
converts AC to DC
converts DC to AC
Speed command signal
Speed control section
Voltage
Hall effect IC (sensor) Speed feedback signal
Speed setting or external DC power supply
Features of the brushless motor ◊ Compact and powerful
Constantly comparing the setup speed and the speed feedback signal from the motor, it adjusts voltage applied to the motor. Even if the load changes, it is capable of maintaining stable operation from high to low speeds.
Compared with the inverter-controlled three-phase motor, the brushless motor is thinner and features higher torque. Since a permanent magnet is used in the rotor, the motor is thinner and features higher power. 50.4 (1.98 in.)
Speed regulation (load)
BMU Series Brushless Motor 120 W (1/6 HP) Mass: 1.2 kg (2.6 lb.)
±0.2%
General inverter (V/f control) Actual speed
(3.31 in.)
Output Power times
Approx. -15% 0
−84 mm
1.3
Inverter Control Three-Phase Induction Motor 90 W (1/8 HP) Mass: 3.2 kg (7.0 lb.)
Command speed: 1000 r/min
*Command speed: 1000 r/min Load factor: 80%
90 (3.54 in.)
Brushless motor BLE Series
90 (3.54 in.)
◊ Stable speed control
135 (5.31 in.)
◊ Wide range speed control
◊ Energy efficiency control
The speed control covers a wider range than the inverter. When the three-phase induction motor is driven by the inverter, torque becomes restricted at low speeds. With no restrictions, the brushless motor is suitable for situations where a constant amount of torque is required from high to low speeds.
A permanent magnet is used for the rotor of the brushless motor to eliminate secondary loss. Compared with an inverter-controlled three-phase induction motor*, it reduces power consumption by about 23%, contributing to energy efficiency. 120
Starting torque
BLE Series
100
0.4
Torque [N•m]
Input (W)
Loss
60
60
Output
BLE Series
Inverter control Three-phase induction motor 60 W
25
60 40
0.2 0.15
20
Continuous Duty Region 0
50
80
Rated torque
Limited Duty Region
Power consumption decreases about 23%
100
1000 2000 Speed [r/min]
0 3000
4000
* When output is 60 W
◊ Variations in power supply and operational interface In our AC input series, with the box-type circuit serving as the base, the following types of brushless motors are available: one allows the speed to be set with a built-in potentiometer and one connects to Factory Automation (FA) networks. In our DC input series, there is a motor with a circuit substrate.
6
AC Input
DC Input
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